8o4 THE POPULAR SCIENCE MONTHLY. 



VARIATIONS IN CLIMATE. 



By W. H. LAERABEE. 



SPECULATIONS concerning changes of climate have an in- 

 terest that never flags. It rarely happens in the succession 

 of seasons that two of an identical character come in succession ; 

 and any more than usually marked variation easily prompts the 

 fancy that some modification in the character of the climate is 

 impending. 



The subject of climatology is a difficult one. The data for the 

 proper study of it have hardly begun to be collected. We are 

 embarrassed when we undertake to define climate and what 

 marks to accept as its characteristics. Hann and Humboldt de- 

 fine it as comprising the whole of the meteorological phenomena 

 characterizing the state of the atmosphere at any place, particu- 

 larly as they affect our organs or have an influence on animal or 

 vegetable life. The general character of the conditions can not 

 be determined by the observations of one year, for they are liable 

 to be contradicted by those of the next year ; nor by those of any 

 short term of years, for a similar reason. A period must be taken 

 long enough to furnish the data for composing a type ; and the 

 more the years vary, as between one another, the longer must the 

 period be. Many factors enter into the composition of a climate 

 and form complicated combinations, all of which must be unrav- 

 eled so as to give each factor its true force and position ; and then 

 the determination of their relative importance affords another 

 source of embarrassment. Temperature and moisture are ac- 

 cepted as the most important factors, and temperature as the 

 dominant one; and the climate is deduced by considering the 

 average mean temperature for a term of years. Equal yearly 

 averages do not, however, signify identical climate. A place 

 where the summer heat and the winter cold are extreme has not 

 the same climate as one where the range is relatively narrow, 

 though the yearly averages may be the same in both. Hence we 

 need separate determinations of summer and winter averages. 

 The combinations of conditions of temperature and moisture may 

 be endless, while the averages of either may be hardly disturbed. 

 These facts make it hard to compare climates even when they are 

 steady for long periods. In the capricious climates of our tem- 

 perate latitudes a just determination and comparison form a 

 baffling task. 



Observations, more or less systematic, with instruments, have 

 been made of climatological features for about a hundred years, 

 but on a general co-operative plan they have been carried on 

 imperfectly for less than a third of that time, or about the period 



